The present invention relates to an apparatus and a method for controlling fuel injection in an internal combustion engine that includes a first fuel injection valve for injecting fuel into a cylinder and a second fuel injection valve for injecting fuel into an intake passage.
Japanese Laid-Open Patent Publication No. 5-231221 discloses an internal combustion engine that has an in-cylinder injection valve for injecting fuel into a cylinder and an intake port injection valve for injecting fuel toward an intake port, and switches the fuel injection mode of each injection valve as necessary. In such an internal combustion engine that has two types of fuel injection valves, the pressure of fuel injected from the in-cylinder injection valve, that is, the pressure of fuel that is supplied to the in-cylinder injection valve, is set higher than the pressure of fuel injected from the intake port injection valve. This is because the in-cylinder injection valve needs to inject fuel against the high pressure in the cylinder, and because fuel needs to be adequately atomized to maintain a favorable combustion state.
In a fuel pressurizing and supplying system for supplying highly pressurized fuel to the in-cylinder injection valve, fuel drawn up from a fuel tank is pressurized by a high pressure pump to a high pressure. The pressurized fuel is supplied to a delivery pipe and then to the in-cylinder injection valve connected to the delivery pipe. In the fuel pressurizing and supplying system, mechanical noise due to supplying of highly pressurized fuel is generated. For example, valve noise, or noise of a valve body hitting a valve seat in a spill valve, in the high pressure fuel pump and similar noise in the in-cylinder injection valve are generated. Such noise generated in the fuel pressurizing and supplying system disturbs a driver of the vehicle particularly when the engine is idling or operated at a low load.
The combustion rate of fuel injected from an in-cylinder injection valve is generally greater than the combustion rate of fuel injected from an intake port injection valve. Therefore, in addition to the mechanical noise generated in the fuel pressurizing and supplying system, combustion noise due to increase in the combustion rate becomes a problem.
Measures for suppressing noise generated in fuel pressurizing and supplying systems and noise due to combustion rate include, for example, a method for lowering the pressure of fuel supplied to an in-cylinder injection valve. Such control for lowering the fuel pressure suppresses the valve noise of a spill valve and that of an in-cylinder injection valve. Further, since such control lowers the pressure of fuel injected from an in-cylinder injection valve, the combustion rate is lowered, and the combustion noise is suppressed, accordingly. Japanese Laid-Open Patent Publication No. 2000-249020 discloses such control for lowering fuel pressure.
Other measures for suppressing combustion noise include a method disclosed in Japanese Laid-Open Patent Publication No. 1-313672, in which the ignition timing of fuel is retarded so that the combustion rate is decreased, and a method disclosed in Japanese Laid-Open Patent Publication No. 11-93731, in which fuel is injected into a cylinder in several times per cycle.
However, if the fuel pressure is lowered to reduce noise in a fuel pressurizing and supplying system and combustion noise, fuel will be insufficiently atomized, and the fuel penetration will be reduced. Therefore, formation of air-fuel mixture is likely to deteriorate, accordingly. As a result, the combustion state can deteriorate. The combustion state when fuel is injected from an in-cylinder injection valve is more susceptible to disturbance than the combustion state when fuel is injected from an intake port injection valve. Therefore, retarding the ignition timing of fuel and injecting fuel in several times per cycle as described above can also cause combustion to deteriorate.